Preheat oven to 350 degrees. Spray a bundt pan liberally with non-stick cooking spray. In a large bowl combine cake mix, pistachio instant pudding mix, eggs, water, and vegetable oil.
Mix with a hand mixer for approximately 2 minutes, blend until smooth, scrape the bowl down occasionally while mixing the cake. Remove 1 cup of cake batter and place in a small bowl, add 1/2 cup chocolate syrup to the 1 cup of cake mix.
Stir the chocolate syrup mixture until the color is uniform. Place yellow cake mix batter into a bundt cake pan, and then pour the chocolate syrup mixture on top of the yellow cake mix in the bundt cake pan.
Using a knife, gently swirl the chocolate and yellow cake mix batters together, do not over stir, you want to have definite marbling. Bake cake in the oven for approximately 55 minutes, you will know the cake is done when a toothpick is inserted it will come out clean.
No icing is needed for this cake. It's perfect the way it is. If you do use an icing or glaze, cut up some fresh pistachios for a little crunch.
Use regular chocolate syrup or change the flavor a bit by using dark chocolate syrup.
If you have any cake leftover, keep in an air-tight container for up to a week.